FIG. 1 shows the rear wheel mounting structure of a conventional horizontal jack, also known as a floor jack. According to this design, the rear wheel mounting structure is formed of a bracket 10 and a shaft 20. The bracket 10 supports a wheel 30. The shaft 20 is inserted through a mounting hole on a rear wheel mount 40 of the horizontal jack and then locked thereto with a locknut 50. The bottom end of the shaft 20 is inserted through a ball bearing 60 at the top side of the bracket 10 and then riveted to the bracket 10. The ball bearing 60 allows free rotation of the bracket 10 relative to the shaft 20.
This design exhibits drawbacks in that it does not allow for the adjustment of the elevation of the bracket 10 and the wheel 30. Continued long-term use of the horizontal jack results in one or more of the four wheels of the jack deviating from a once-common horizontal plane. When this occurs, the wheels become unbalanced. As a result, they may damage the ground, interfere with the movement of the jack, or produce a loud noise when the jack is moved on the ground. Additionally, when supporting a load, one or more of the wheels opposite the point of contact with the load on the lift arm of the jack, may move out of contact with the ground, destabilizing the lift, resulting in a dangerous situation.